Plastic Pipes Conference Association # 2008 Budapest
Lang, Frank, Pinter
For lifetime prediction of pressurized polyethylene (PE) pipes based on methods of the linear elastic fracture mechanics the knowledge of the crack resistance and kinetics of creep crack growth (CCG) is essential. In the present work a rather brittle non pipe material was used to develop a methodology for an accelerated measurement of crack kinetics in fatigue tests on cracked round bar (CRB) specimens at ambient temperatures of 23 °C. A material and specimen specific compliance calibration curve was generated to detect the crack kinetics with only one single CRB test. Based on an already proposed concept the kinetics at different R-ratios (minimum/maximum load) were measured and extrapolated to the case of CCG. To demonstrate the transferability of this concept to pipe materials a PE 80 pipe grade was used. Although the necessary testing time increased considerably, the concept still has the potential to reduce the overall testing time for new pipe materials to be certified significantly (up to a maximum of a few months). Also it is important to mention that the cyclic CRB tests proved to be an excellent method for quick material ranking and material development. Thus, testing times for failure as a result of slow crack growth could be reduced to one week at room temperature and without the use of stress cracking liquids for modern PE pipe grades.
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